Usage of "one of" if a relative pronoun whose is used in the sentence what will be the verb?
One of the students whose mobile (is/are) missing
What is missing in this sentence? Is it singular, or plural? If it is singular, then you need is missing, and if it is plural, you want are missing.
As you have written it, the thing that is missing is mobile, so you must have is missing.
Whether you should be saying mobile is or mobiles are is a whole other question, and one that I've definitely known native speakers to differ on. Personally, I'd make it mobiles are, because there is more than one mobile phone involved.
One of in this case doesn't change anything, because it's "one of (the students whose mobile is missing)". However, you could have:
One of the students' mobiles is missing
There you can see that one of causes an expression whose grammatical number is plural, "the students' mobiles" and makes it singular by indicating that the reference is to just one of them, "(one of (the students' mobiles)) is missing".
One of the students [whose mobile is missing] is very upset.
Your example was just a noun phrase, so I've added the predicate "is very upset" to produce a complete sentence.
You've used singular "mobile" , so I'll assume that just one mobile is missing.
The one of x who construction often causes confusion since either singular or plural agreement is possible depending on the intended meaning.
Here, the natural interpretation is that it's not a matter of there being a set of students whose mobiles are missing, but of there being one student whose mobile is missing.
The relative clause thus belongs in the topmost noun phrase, not the one with "students" as head. Which means that the verb in the main clause and the one in the relative clause should be the singular "is", as shown.
The sentence is incomplete because you haven't finished talking about the main subject—the student. You actually need to have two verbs, each of which will be formed based on its own subject.
There are various possibilities, involving one or more students and there being one or more phones owned by those students:
One of the students [whose mobile was missing] was distraught.
One of the students [whose mobiles were missing] was distraught.
Two of the students [whose mobiles were missing] were distraught.
It's possible to describe a situation where all of the students had multiple missing phones, but the language becomes more complicated:
Two of the students, [each of whom whose mobiles were missing], were distraught.
The only way to make it clear that multiple phones belonged to every student is to use each, in order to make it clear that mobiles is being used for individuals.